The present invention relates to a YC separation circuit for separating a luminance (Y) signal and a chrominance (C) signal from image signals by means of a comb filter, and more specifically, to a YC separation automatic adjusting circuit having a function for automatically adjusting a signal separation.
There is described first a conventional YC separation circuit in reference with FIG. 1 showing an example of a so-called comb filter. The comb filter uses characteristics of carrier color signals in which a phase is inverted by 180 degrees each horizontal scanning so as to separate a luminance signal and a chrominance signal on the basis of the sum and difference of two signals having a time difference corresponding to one horizontal scanning time (1H). In this example, a level of a delay image signal is adjusted in the manner that amplitudes of two image signals are equal to each other, and a shift amount of phase of the delay image signal is adjusted in the manner that the phase difference between two image signals becomes an exact 1H. By adding with such adjusting, chrominance signals are interposed with the same amplitude and the phase difference of 180 degrees each other at an addition and subtraction of two image signals, namely, the luminance signal is separated by the addition and the chrominance signal is separated by the subtraction.
In detail, in FIG. 1, an image signal is supplied to an input terminal 1 from a tuner or a video tape recorder (not shown). The image signal is delayed for a time corresponding to one horizontal scanning time by means of a delay element 5 for delaying a signal such as a charge coupled device (abbreviated in a CCD hereafter) and the like so as to be a delayed image signal which is supplied through a variable gain circuit 6 and a variable phase shift circuit 7 to each one of input terminals of a subtracter 10 and an adder 11, respectively. There is supplied the image signal which is not passed through the delay element 5, to each of the other input terminals of the subtracter 10 and the adder 11, respectively. The subtracter 10 subtracts the delayed image signal from the image signal both of which phases of respective chrominance signals are different from 180 degrees each other, thereby offsetting a luminance signal component to output a chrominance signal to an output terminal 2. The adder 11 adds the delayed image signal with the image signal both of which phases of respective chrominance signals are different from 180 degrees each other, thereby offsetting a chrominance signal to output a luminance signal to an output terminal 3.
The image signal supplied to the input terminal 1 and the delayed image signal outputted from the variable phase shift circuit 7 are supplied to a level detection circuit 8 and a phase detection circuit 9. To both of the detection circuit 8 and 9, a burst gate pulse is supplied corresponding to a position of a color burst signal interposed on the image signal, the color burst signal which is supplied through another input terminal as a burst gate pulse input terminal 4.
The level detection circuit 8 comprises a gate circuit which is controlled with respect to an "ON" condition by a burst gate pulse, a pair of full-wave rectification circuits, and a pair of low-pass filters.
One of the level detectors detects a level of the color burst signal in the image signal, while the other level detector detects a level of the color burst signal in the delayed image signal. Furthermore, detected two signal levels are supplied to a differential amplifier (not shown in the figure) which detects the difference between the levels. The level difference is supplied to the variable gain circuit 6 as a gain control signal for adjusting the levels of the image signal and the delayed image signal respectively supplied to each of the input terminals of the subtracter 10 and the adder 11 in the manner that the levels of both of the signals are equal to each other.
The phase detection circuit 9 comprises a pair of gate circuits, a pair of 90-degree phase shifters, a pair of multipliers, and a pair of low-pass filters which smooth outputs of the multipliers, respectively. The image signal is supplied to one of input terminals of the multiplier through the gate circuit and the 90-degree phase shifter on one side, while the delayed image signal is supplied to the other of input terminals of the multiplier through the gate circuit and the 90-degree phase shifter on the other side. The phase detection circuit 9 corresponds to the supply of the burst gate pulse, compares phases of the color burst signals of the image signal and the delayed image signal, respectively, and supplies an error signal as a phase shift adjusting signal to the variable phase shift circuit 7 corresponding to the phase difference of the image signal and delayed image signal, thereby adjusting the phase difference to be kept to 180 degree between the color signals of two image signals which are supplied to the subtracter 10 and the adder 11, respectively.
An ensure of a detecting precision of the level detection circuit and the phase detection circuit is essential to operate the above-mentioned YC separation automatic adjusting circuit along the purpose of the provision thereof. It is generally required that a signal separation performance should be more than -30 dB in the YC separation. Therefore, the above-mentioned level detection circuit and phase detection circuit are required to have a high detecting precision.
However, it is quite difficult to precisely make the 90 degree phase shift circuit which uses in the phase detector. Furthermore, even though an amplitude and a phase of the image signal and the delayed image signal which are supplied to the adder and subtracter, coincide with each other, there are a discrepancy of the gain with respect to two input signals into the adder and subtracter and a discrepancy of the signal delay time, thereby reducing a signal noise ratio (S/N) of the signal separation caused by the addition and subtraction of the image signal and delayed image signal.